Ion traps are quadrupole mass analyzers used to trap, store, and sort charged particle ions by their mass, and serve as the basis of ion trap mass spectrometer instruments. In general, quadrupole ion traps (QIT) or Paul ion traps are devices comprised of three electrodes creating an electric field equipotential well at their central axis for trapping, storing, and sorting ions. This is accomplished through applied radio frequency (RF) and direct current (DC) voltages to the central ring and end cap electrodes of the trap, respectively.
Ion traps are a type of mass spectrometer used for molecular sample identification. Cylindrical ion traps (CIT) are a simplified version with two end plates and one central ring electrode with flat plate geometry. Ion traps and CITs operate by containing or “trapping” charged particles in a symmetric oscillating electric field for subsequent analysis by increasing the magnitude of the oscillating field. Analysis of the sample ions (analyte) is done by axial ejection of the ions, where the ion signal is recorded by a detector. However, the axial ion ejection occurs in equal and opposite directions along the ejection axis, so about half of the sample analyte is typically lost in analysis. That is, existing QITs and CITs are symmetric devices where half (or approximately half) of the mass sorted ions are lost (see, for example, March et al., Quadrupole storage mass spectrometry, Chemical analysis, 1989, New York: Wiley, page 471). As described by March et al, the general equation describing the electric field potential well of the ion trap and ion motion within is,
                    ∅        =                                            1              2                        ⁢                                          (                                  U                  -                                                            V                      ⁢                      cos                      ⁢                      Ω                                        ⁢                                                                                  ⁢                    t                                                  )                            ⁡                              [                                                                            x                      2                                        +                                          y                      2                                        -                                          2                      ⁢                                                                                          ⁢                                              z                        2                                                                                                  r                    o                    2                                                  ]                                              +                      [                                          U                -                                                      V                    ⁢                    cos                    ⁢                    Ω                                    ⁢                                                                          ⁢                  t                                            2                        ]                                              (        1        )            with U representing the DC voltage, and V the RF voltage, applied to the end and central ring electrodes respectively (March et al., Practical aspects of ion trap mass spectrometry, Modern mass spectrometry, 1995, Boca Raton, Fla.: CRC Press). The functional radius of the central ring electrode is ro, and Ω is the frequency of the applied RF voltage.